Present day apparatus for the calcination of alumina may be subdivided in three groups as follows:
(a) Drying and preheating of the aluminium-oxide-hydrate obtained from bauxite is carried out in cyclones whereas annealing takes place in rotary or revolving furnaces. The product obtained thereby is cooled in planet coolers, cooler drums and the like.
(b) Both drying and preheating of the hydrate as well as cooling of the product take place in cyclones while annealing is carried out in rotary or revolving furnaces.
(c) Drying, preheating and heating is carried out in cyclones whereas annealing takes place in special reactors. Cooling of the product is carried out by cyclones and fluidisation coolers.
Though alumina consisting entirely of Gamma modification is particularly suitable for metallurgical purposes, practically, such concentration cannot be obtained and, therefore, products are aimed at which--in addition to the Gamma modification--contain only Alpha modification and that in relatively small amounts with respect to the Gamma modification.
The alumina calcination apparatus of the groups (a) and (b) have the common characteristic feature that the calcined alumina produced thereby comprises the entire spectrum of the various crystalline modifications. This is due to the hydrate passing the heating means very quickly and within a time interval of 1 to 2 minutes during which it is heated to a temperature of 400 to 500 degrees centigrade. A further heating to a temperature of 1200 degrees centrigrade is carried out in rotary or revolving muffle furnaces which work under very favourable operational conditions. The dwell time of the work material in such furnace can be altered by means of changing the speed of furnace rotation. Nevertheless, some particles or grains of the work material may fail to be heated to desired temperatures or the dwell time required for thermal decomposition may be unduly short though the temperature is of the desired value.
Such inconveniences are absent with apparatus of the third group (c) of the known alumina calcination apparatus comprising a reactor in which more suitable thermal conditions for annealing can be obtained. In addition, the dwell time of the work material may be adjusted within wide limits. As a result, alumina of a suitable proportion of Gamma and Alpha modifications can be produced. However, the alumina calcination apparatus of the group (c) require an intricate automatic unit for discharging the produced alumina.